In a relay station system of the related art, when both end users U1 and U2 exchange information using a relay station R, as known from FIG. 1, a total of four data transmission processes are required.
In order to reduce the number of four transmission processes, a two-way relay station system using network coding has been in the spotlight.
As illustrated in FIG. 2, in the above system, the users U1 and U2 simultaneously transmit the individual information to the relay station during a first transmission process and the relay station receives the information to apply network coding. In a second transmission process, data produced by doing this is simultaneously transmitted to the users U1 and U2. The users U1 and U2 remove information which is transmitted by themselves and already known from the received information to obtain information of the other party.
The first transmission process is referred to as a multiple access (MA) process and the second transmission process is referred to as a broadcast (BC) process.
The network coding method which is applied in the relay station includes an analog network coding (ANC) and a physical network coding (PNC).
According to the ANC, the relay station adjusts power of the signals received from both users to generate network coding information which will be transmitted in the BC process and according to the PNC, the relay station decodes information received from both users to generate network coding information which will be transmitted during the BC process using an operation such as an exclusive logic sum (XOR).
However, the ANC has restrictions that the user requires not only channel information between the user and the relay station but also channel information between the other user and the relay station and a drawback in that noise is increased.
However, as compared with the ANC, in the PNC, the user needs to know only a channel between the user and the relay station and the signal is decoded in the relay station so that there is no increase in noise.
According to a beam forming method of a two-way relay system in the related art, it is designed without considering the PNC so that in the PNC, maximum performance is not obtained or even though the PNC is considered, it is not optimized to maximize a maximum QoS of both users.